Electrode for secondary battery and manufacturing method thereof

ABSTRACT

A battery technology, and more particularly, a current collector may be widely used in secondary batteries, and an electrode may employ such technology. The current collector includes a conductive fiber layer including a plurality of conductive fibers. Each of the conductive fibers includes a conductive core including a plurality of metal filaments, and a conductive binder matrix surrounding the outer circumferential surfaces of the conductive core.

TECHNICAL FIELD

The present invention relates to a secondary battery technology, andmore particularly, to an electrode for a binder-free secondary batteryand manufacturing method thereof.

BACKGROUND ART

A secondary battery is a chargeable and rechargeable battery employing afine electrode material, where a representative example ofcommercialization thereof is a lithium secondary battery. The lithiumsecondary battery is expected to be applied as a small power source fora small IT device, such as a smart phone, a portable computer, and anelectronic paper, but also as a mid-sized or large-sized power sourcemounted on a means of transportation, such as a car, or used in a powerstorage in a power supply network, such as a smart grid.

If lithium metal is used as an anode electrode material of a lithiumsecondary battery, the battery may be short-circuited or explode due toformation of dendrite. Therefore, a crystalline carbon, such as graphiteor artificial graphite, soft carbon, hard carbon, or a carbon-basedactive material capable of intercalating and deintercalating lithium iscommonly used instead of the lithium metal for an anode electrode.However, as fields of application of a secondary battery expand, it isdemanded to further increase capacity and output power of a secondarybattery, and thus non-carbon-based anode electrode materials that havecapacities of 500 mAh/g or higher and may be alloyed with lithium, suchas silicon (Si), tin (Sn), or aluminum (Al), are being spotlighted asmaterials for replacing carbon-based anode electrode materials having atheoretical capacity of 372 mAh/g.

From among the non-carbon-based anode electrode materials, siliconexhibits the largest theoretical capacity of about 4,200 mAh/g, and thusutilization of silicon is very important in terms of capacity. However,volume of charged silicon is about 4 times greater than that ofdischarged silicon. Therefore, due to change of volume during chargingand discharging operations, electric connection between active materialsis destroyed, an active material is detached from a current collector,and an irreversible reaction deteriorating life expectancy of a battery,such as formation of a solid electrolyte interface (SEI) layer like aLi₂O layer due to corrosion of the active material due to anelectrolyte, occurs, and thus it is difficult to utilize silicon as aanode electrode material.

To overcome the problems, a method of fabricating a nano-size activematerial, a method of enhancing surfaces of an active material by usinggraphene or carbon, and a method of synthesizing materials havingvarious structures have been suggested. Furthermore, a technique forresolving problems of a anode electrode based on charging/dischargingoperations by forming a plurality of grooves with semicircular crosssections or a plurality of hemispherical grooves by wet-etching asurface of a current collector foil or forming nano-wires grown to havefirst ends substantially fixed to a current collector foil on an upperportion of the current collector foil and depositing an active materialthereto via a physical vapor deposition has been suggested. However, inthis case, since an active material layer covering all of the nano-wiresis formed, and thus the active material may be exfoliated and it isdifficult to effectively increase a ratio of specific surface area tovolume due to the nano-wires.

DISCLOSURE OF THE INVENTION Technical Problem

The present invention provides an electrode for a secondary battery, theelectrode for suppressing irreversibility based on change of volumeduring charging and discharging operations and effectively increasing aratio of specific surface area to volume for commercialization of a newhigh capacity active material with a large volume expansion ratio.

The present invention also provides a method of fabricating an electrodefor a secondary battery having the above-stated advantages.

Technical Solution

According to an aspect of the present invention, there is provided anelectrode for a secondary battery, the electrode including a non-wovenfabric current collector including metal fibers that form continuousporosity from a surface of the non-woven fabric current collector to theinterior of the non-woven fabric current collector; and an activematerial layer deposited onto the metal fibers in a non-radial shape viathe porosity in a plasma-based sputtering operation.

According to an embodiment, the active material layer deposited in thenon-radial shape has a circularity, which is defined by Equation 1below, from 0.2 to 0.8;

$\begin{matrix}{{Circularity} = \frac{2\sqrt{\pi \; A}}{P}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(A denotes an entire area of a cross-section of the metal fiber and theactive material layer formed on the metal fiber, and P denotescircumferential length of the cross-section). The cross-section has anelliptical shape.

According to an embodiment, the active material layer is deposited froma surface of the non-woven fabric current collector to the interior ofthe non-woven fabric current collector. In this case, size of theporosity is equal to or larger than that of the sheath of the plasma.Size of the porosity is within a range from about 0.01 mm to about 2 mm.

According to an embodiment, diameter of the metal fiber is within arange from about 1 μm to about 200 μm. Furthermore, the metal or themetalloid is any one selected from a group consisting of tin (Sn),silicon (Si), antimony (Sb), zinc (Zn), germanium (Ge), aluminum (Al),copper (Cu), bismuth (Bi), cadmium (Cd), magnesium (Mg), cobalt (Co),arsenic (As), gallium (Ga), lead (Pb), and iron (Fe) or aninter-metallic compound.

The metal fiber is formed of a stainless steel, iron, aluminum, copper,nickel, chromium, titanium, vanadium, tungsten, manganese, cobalt, zinc,ruthenium, lead, iridium, antimony, platinum, silver, gold, or acompound thereof. The metal fibers are acid-cleaned to control surfaceroughness.

According to an aspect of the present invention, there is provided amethod of fabricating an electrode for a secondary battery, the methodincluding providing a non-woven fabric current collector including metalfibers, which form porosity, in a plasma reactor; providing a sputteringtarget including a metal, a metalloid, an oxide thereof, or a mixturethereof including an active material in the plasma reactor; anddepositing an active material layer on the metal fibers in a non-radialshape via the porosity in a plasma-based sputtering operation.

According to an embodiment, the active material layer deposited in thenon-radial shape has a circularity, which is defined by Equation 1below, from 0.2 to 0.8;

$\begin{matrix}{{Circularity} = \frac{2\sqrt{\pi \; A}}{P}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

(A denotes an entire area of a cross-section of the metal fiber and theactive material layer formed on the metal fiber, and P denotescircumferential length of the cross-section).

The non-woven fabric current collector is levitated inside the plasmareactor, such that all surfaces facing the main surfaces of thenon-woven fabric current collector are exposed to plasma. The activematerial layer is deposited from a surface of the non-woven fabriccurrent collector to the interior of the non-woven fabric currentcollector. The interior of the plasma reactor includes an oxidizingatmosphere or a reducing atmosphere.

Size of the porosity is equal to or larger than that of the sheath ofthe plasma. According to an embodiment, size of the porosity is within arange from about 0.01 mm to about 2 mm. Diameter of the metal fiber iswithin a range from about 1 μm to about 200 μm.

The metal or the metalloid is any one selected from a group consistingof tin (Sn), silicon (Si), antimony (Sb), zinc (Zn), germanium (Ge),aluminum (Al), copper (Cu), bismuth (Bi), cadmium (Cd), magnesium (Mg),cobalt (Co), arsenic (As), gallium (Ga), lead (Pb), and iron (Fe) or aninter-metallic compound. The metal fiber is formed of a stainless steel,iron, aluminum, copper, nickel, chromium, titanium, vanadium, tungsten,manganese, cobalt, zinc, ruthenium, lead, iridium, antimony, platinum,silver, gold, or an compound thereof.

Advantageous Effects

According to an embodiment of the present invention, by depositing anactive material layer onto metal fibers of a non-woven fabric currentcollector including the metal fibers, which form porosity, in anon-radial shape by operingusing a plasma-based sputtering, an electrodefor suppressing tensile stress formed during operations for charging anddischarging a high capacity active material with a large volumeexpansion ratio to suppress irreversibility, such as exfoliation of theactive material, and to effectively increase a ratio of specific surfacearea to volume for increased energy density may be provided.

Furthermore, according to another embodiment of the present invention, amethod of fabricating an electrode for a secondary battery, thebinder-free method for dry-fabricating an electrode including anon-woven fabric current collector having formed thereon an activematerial layer in a non-radial shape may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of an entire non-woven fabric type currentcollector according to an embodiment of the present invention.

FIGS. 2A and 2B are diagrams showings cross-sectional structures ofmetal fibers having deposited thereon an active material layer and anactive material layer according to embodiments of the present invention.

FIGS. 3A and 3B are diagrams showing methods of forming electrodes usingsputtering, according to embodiments of the present invention.

FIGS. 4A to 4C are diagrams showing stages of growth of a lithiatedlayer that an active material formed on a metal fiber in a non-radialshape experiences as being lithiated during a charging operationaccording to an embodiment of the present invention, and FIG. 4D is agraph showing change of stresses in the respective stages of growth.

FIGS. 5A to 5C are diagrams showing stages of growth of a lithiatedlayer that an active material formed on a metal fiber in a radial shapeexperiences as being lithiated during a charging operation according toa comparative embodiment, and FIG. 5D is a graph showing change ofstresses in the respective stages of growth. The horizontal axisindicates charging times corresponding to lithium insertion, whereas thevertical axis indicates stresses.

FIG. 6 is a sectional view for describing an electrochemical reaction ofa battery cell 500 employing an electrode according to an embodiment ofthe present invention.

FIG. 7 is a diagram showing a silicon active material layer depositedonto the metal fiber in a non-radial shape.

FIGS. 8A and 8B are graphs showing discharging characteristics measuredby applying constant currents of 300 mA/g to the battery according tothe first embodiment and the battery according to the first comparativeembodiment, respectively.

FIG. 9 is a graph showing a result of evaluating life expectancycharacteristics of the battery according to the first embodiment and thebattery according to the first comparative embodiment after beingcharged and discharged 200 times by applying currents of 2,000 mA/gthereto.

FIGS. 10A and 10B are scanning electron microscope images of the batteryaccording to the first embodiment before being charged/discharged andafter being charged/discharged.

FIG. 11 is an optical image of the battery according to the firstcomparative embodiment before being charged/discharged and after beingcharged/discharged.

MODE FOR CARRYING OUT THE INVENTION

Hereinafter, exemplary embodiments will be described in detail withreference to accompanying drawings.

The invention may, however, be embodied in many different forms andshould not be construed as being limited to the embodiments set forthherein; rather, these embodiments are provided so that this disclosurewill be thorough and complete, and will fully convey the concept of theinvention to those skilled in the art.

Furthermore, in the drawings, the thicknesses of layers and regions areexaggerated for clarity, and like reference numerals in the drawingsdenote like elements. As used herein, the term “and/or” includes any andall combinations of one or more of the associated listed items.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

When applied to a lithium secondary battery, next generation anodeelectrode active materials, such as tin (Sn), silicon (Si), antimony(Sb), zinc (Zn), germanium (Ge), aluminum (Al), copper (Cu), bismuth(Bi), cadmium (Cd), magnesium (Mg), arsenic (As), gallium (Ga), lead(Pb), and iron (Fe), exhibit significant volume changes based onrepeated charging and discharging operations. For example, in case ofsilicon anode electrode active material, a reaction that a Li_(x)Sicompound is formed based on an electrochemical reaction between lithiumions and the silicon anode electrode active material occurs from asurface of the silicon anode electrode active material. In this case, anacute interface is formed between pristine-Si and a lithium compound(Li_(x)Si) layer. As lithiation proceeds, the lithium compound(Li_(x)Si) layer becomes larger. When the entire silicon particles arechanged to Li_(x)Si compound, the electrochemical reaction isterminated.

A silicon interior layer not yet reacted during the lithiation and alithium compound layer exists in a silicon anode electrode activematerial layer. As lithiation proceeds, tensile hoop stress is appliedto the lithium compound layer at a certain time point at which thelithium compound layer surrounds silicon particles. The tensile hoopstress is the main reason of surface cracks and destruction of siliconparticles.

However, since an anode electrode active material, such as a silicon, ismore resistant to a compression stress than to a tensile stress, even ifa compression stress that is 10 time or greater than a tensile hoopstress is applied, surface of an active material layer is barely crackedor broken. Therefore, the inventors of the present invention inventedthe present invention for preventing surface cracks of an anodeelectrode active material layer by preventing or minimizing applicationof a tensile hoop stress to surfaces thereof during a lithiation.

According to embodiments of the present invention, by suppressing andreducing the tensile hoop stress by controlling circularity of across-section of a anode electrode active material layer by using asputtering method, formation of cracks based on volume expansion thatoccurs during lithiation and irreversible deterioration of lifeexpectancy may be effectively enhanced. The embodiments below relate toan electrode for a secondary battery formed by depositing an activematerial layer with a large volume expansion ratio onto the metal fibersof a non-woven fabric current collector, which includes the metal fibersthat form porosity by being atypically and 3-dimensionally arranged, viaa sputtering operation and a method of fabricating the electrode.

In the present specification, a metal fiber refers to a linear structureformed by fiberizing a metal, such as a stainless steel, aluminum,nickel, titanium, copper, or an alloy thereof. The metal fiber exhibitscharacteristics of a metal, such as heat resistance, plasticity, andelectroconductivity, and exhibits fibrous characteristics for anon-woven fabric processing operation. The present invention relates tocharacteristics and advantages for applying the characteristics of themetal fiber to an electrode structure of a battery.

The metal fibers may be fabricated by maintaining a metal or a compoundin a suitable container in the form of a molten liquid and rapidlysolidifying the molten liquid by jetting the molten liquid into the airthrough discharge holes of the container by using a compressed gas or apressing device, such as a piston. Alternatively, the metal fibers maybe fabricated by using a bundle drawing method known in the art. Bycontrolling a number and size of the discharge holes and/or scatteringof discharged molten metal, thickness, uniformity, tissue like non-wovenfabric, and aspect ratio of metal fibers may be controlled. Metal fibersconstituting a battery according to the present invention may includenot only metal fibers fabricated by using the above-stated fabricationmethods, but also metal fibers fabricated by using other methods knownin the art, where the present invention is not limited thereto.

The term ‘separator’ used in the present specification includes aseparator commonly used in a liquid electrode battery employing a liquidelectrode exhibiting a small affination with respect to the separator.Furthermore, the ‘separator’ in the present specification also includesan intrinsic solid polymer electrolyte and/or a gel solid polymerelectrolyte in which an electrolyte is strongly bound to the separatorand the electrolyte and the separator are considered as a singleelement. Therefore, definition of the separator should be set forth asdefined in the present specification.

FIG. 1 is a perspective view of an entire non-woven fabric type currentcollector 10 according to an embodiment of the present invention

Referring to FIG. 1, the non-woven fabric type current collector 10includes a metal fiber 10W constituting porosity 10P. The metal fiber10W may be segmented to have a suitable length and the plurality ofmetal fibers 10W may be arranged. According to embodiments of thepresent invention, lengths and a number of the metal fibers 10W may besuitably selected based on size and capacity of a battery. For example,the metal fiber 10W may have a thickness from about 1 μm to about 200 μmand a length from about 5 mm to about 1000 mm, and thus the metal fiber10W may have an aspect ratio from about 25 to about 106.

For example, in case of the non-woven fabric type current collector 10formed as a three-dimensional structure consisting of the metal fibers10W having a diameter of 10 μm, surface area thereof is from about 13cm² when the non-woven fabric type current collector 10 has a circularshape with a diameter of 12 mm, where the surface area of the non-wovenfabric type current collector 10 is about 6 times greater than surfacearea of a metal thin-film current collector having a same weight (about2 cm²). Therefore, in relationship with an active material layerdeposited thereon, a low-resistance interface is formed due to theincreased area, and thus internal resistance thereof may besignificantly reduced.

FIG. 1 shows that the metal fibers 10W have substantially linear shapeand curved shapes. However, according to another embodiment of thepresent invention, the metal fiber 10W may also be formed to have any ofvarious other regular or irregular shapes, such as a curled shape, aspiral-like shape, etc.

The metal fibers 10W are electrically connected to one another by beingphysically or chemically combined with one another, thereby forming asingle conductive network. According to embodiments of the presentinvention, the metal fibers 10W may form a non-woven fabric structure bybeing randomly arranged and combined with one another as shown inFIG. 1. The metal fibers 10W are curved or bent and are tangled with oneanother, thereby forming a conductive network having porosity 10P with alow resistance, and a high mechanical strength. The porosity 10P mayform a path in which a fluid may flow from a surface of the non-wovenfabric type current collector 10 into the interior of the non-wovenfabric type current collector 10.

The metal fiber 10W may be formed of a stainless steel, iron, aluminum,copper, nickel, chromium, titanium, vanadium, tungsten, manganese,cobalt, zinc, ruthenium, lead, iridium, antimony, platinum, silver,gold, or an alloy thereof and may preferably be any one of a stainlesssteel, iron, aluminum, copper, nickel, chromium, titanium, platinum,silver, gold, and an alloy thereof. According to an embodiment of thepresent invention, the metal fiber 10W may contain two or more differenttypes of metals. According to embodiments of the present invention, themetal fibers 10W may be chemically combined with one another throughformation of an inter-metallic compound between the two or more metalsby performing an additional operation, such as a heat treatment. Themetal fibers 10W may be acid-cleaned to increase surface roughness forimproved adhesiveness to an active material layer.

Since the metal fibers 10W constituting the conductive network may bemechanically independent, capable of forming a structure, and functionas a current collector by relying on one another, thus beingdistinguished from particle-like linear structures, such asnano-structures or carbon nanotubes linearly grown on a currentcollector foil in the related art.

FIGS. 2A and 2B are diagrams showings cross-sectional structures ofmetal fibers 10W having deposited thereon the active material layer 100Aand the active material layer 100B according to embodiments of thepresent invention.

Referring to FIG. 2A, the active material layer 100A may be formed asreactive species or ion species for an active material diffused ordrifted through porosity (10P of FIG. 1) in a sputtering operation usingplasma are deposited on the metal fibers 10W. Such a dry depositionoperation is a binder-free operation, and thus reduction of internalresistance due to a binder and introduction of a conductive material maybe eliminated.

The active material layer 100A may be formed on the metal fibers 10Wfrom surfaces of a non-woven fabric type current collector (10 ofFIG. 1) to the interior thereof. For the ion species to be sufficientlydrifted to the interior of the non-woven fabric type current collector10, size of porosity (diameter of a sphere that may be passed throughthe porosity) may be equal to or greater than a plasma sheath inducedunder reaction conditions of an active material layer during asputtering operation. According to an embodiment of the presentinvention, size of the porosity 10P may be from about 0.01 mm to about 2mm.

According to an embodiment of the present invention, deposition occursby ion species that are accelerated by an electric field based on plasmain a constant direction, e.g., a direction from bulk plasma toward thenon-woven fabric type current collector 10, the active material layer100A is formed on the metal fibers 10W more smoothly along a particulardirection. As indicated by the arrow G, the active material layer 100Aformed on the metal fiber 10W to have greater thickness upward. Thedirection indicated by the arrow G may be a direction opposite to adirection of an electric field between plasma and the non-woven fabrictype current collector 10.

Referring to FIG. 2B, the active material layer 100B formed on the metalfiber 10W is grown to have a cross-sectional shape having greaterthickness in two opposite directions from the metal fiber 10W asindicated by the arrows G1 and G2. The active material layer 100B may begrown in two directions by rotating a direction in which the non-wovenfabric type current collector 10 is arranged by 180 degrees during asputtering operation for deposition of the active material layer 100B.Detailed descriptions thereof will be given below with reference to FIG.3B.

As described above with reference to FIGS. 2A and 2B, the activematerial layers 100A and 100B are formed on the metal fibers 10W to havenon-radial shapes. The non-radial shapes of the active material layers100A and 100B may be evaluated based on circularity as defined byEquation 1 below. The circularity is determined as a ratio of an entirecross-sectional area of the active material layer 100A or 100B to acircumferential length of the active material layer 100A or 100B.

$\begin{matrix}{{Circularity} = \frac{2\sqrt{\pi \; A}}{P}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$

Here, A denotes an entire area of a cross-section of the metal fiber andthe active material layer formed on the metal fiber, and P denotescircumferential length of the cross-section. The circularity may bemeasured from an image obtained from a scanning electron microscope byusing commercial software like ImageJ®, e.g., Imagej136. Alternatively,the circularity may be measured by using a flow particle image analyzerlike FPIA-3000® of SYSMEX Co. Ltd (Kobe, Japan).

The circularity may be within a range from about 0.2 to about 0.8. Ifthe circularity is smaller than 0.2, micronization of an active materiallayer may begin from a region of thin deposition due to a plurality ofcharging and discharging operations, and thus life expectancy of theactive material layer may be deteriorated. On the contrary, if thecircularity exceeds 0.8, the active material layers 100A and 100B may beeasily cracked or fractured due to tensile stresses applied to lithiatedlayers. Formation of a SEI layer on inner surfaces of the activematerial layer that are exposed by the crack or the fracture may beaccelerated, thereby causing deterioration of life expectancy of abattery.

The active material may contain a metal, a metalloid, an oxide thereof,or a mixture thereof that electrochemically reacts with lithium ions viaalloying/dealloying, occlusion/release, absorption/desorption, or acombination thereof of the lithium ions. For example, the metal or themetalloid may be any one selected from a group consisting of tin (Sn),silicon (Si), antimony (Sb), zinc (Zn), germanium (Ge), aluminum (Al),copper (Cu), bismuth (Bi), cadmium (Cd), magnesium (Mg), cobalt (Co),arsenic (As), gallium (Ga), lead (Pb), and iron (Fe) or aninter-metallic compound. However, the above-stated materials are merelyexamples, and the present invention is not limited thereto.

FIGS. 3A and 3B are diagrams showing methods of forming electrodes usingsputtering, according to embodiments of the present invention.

Referring to FIG. 3A, a plasma reactor 1000 may be a capacitivelycoupled reactor including an anode AE and a cathode CE. A sputteringtarget TG is arranged inside the plasma reactor 1000. The sputteringtarget TG may be a plastic body or a sintered body of a metal, ametalloid, an oxide, or a mixture thereof including the above-statedactive material. The non-woven fabric type current collector 10 may bearranged to face the sputtering target TG. An inert discharging gas,such as argon gas, is introduced into the plasma reactor 1000 (arrow A)at a controlled flux and the interior of the plasma reactor 1000 isexhausted, thereby maintaining the interior of the plasma reactor 1000at a constant pressure. According to some embodiments, a reactive gas,such as an oxidizing gas (e.g., oxygen gas or ozone gas) or a reducinggas (e.g., nitrogen gas or hydrogen gas), may be further supplied. Next,when an alternated power supply RF electrically combined with thecathode CE is turned on, a gas discharge is induced inside the plasmareactor 1000, and thus plasma PL is formed.

The plasma PL forms an electric field in a direction from the anode AEtoward the cathode CE as indicated by the arrow K, and clusters, neutralspecies, or ion species of an active material desorbed from thesputtering target TG are transferred toward the non-woven fabric typecurrent collector 10 and deposited onto metal fibers (10W of FIG. 1) ofthe non-woven fabric type current collector 10. Linearity of theclusters, the neutral species, or the ion species may be controlled byadjusting flux, pressure, power intensity, and an interval betweenelectrodes inside the plasma reactor 1000, and thus circularity of anactive material layer deposited onto metal fibers may be controlled. Ifthe linearity increases, the circularity may decrease, and vice versa.Furthermore, according to an embodiment of the present invention, ifsize of porosity of the non-woven fabric type current collector 10 isequal to or greater than that of the sheath of the plasma PL, linearityof ion species may be maximized, and thus circularity may be reduced.

Referring to FIG. 3B, the non-woven fabric type current collector 10 maybe levitated inside the plasma reactor 1000, such that the main surfacesof the non-woven fabric type current collector 10, that is, the uppersurface 10U and the lower surface 10B facing each other are exposed tothe plasma PL. To this end, a supporting member for fixing an end of thenon-woven fabric type current collector 10 may be provided inside theplasma reactor 1000.

In the method of fabricating an electrode using sputtering according tothe present invention, although not limited, working pressure inside theplasma reactor 1000 is within a range from about 10⁻³ Torr to about 10⁻⁷Torr, may be within a range from about 10⁻⁴ Torr to about 10⁻⁶ Torr, andmay preferably be about 10⁻⁶ Torr. Generally, in a sputtering operation,as the working pressure increases, scatterings of neutral species, ionspecies, and reactive species increase, and thus an electrode withuniformly formed contacts may be formed. However, if the workingpressure exceeds 10⁻³ Torr, sputtered ions are excessively scattered. Asa result, density of ion species becomes excessively high duringdeposition, and thus thermodynamic energy may be lost. Incidentally, ifthe working pressure is lower than 10⁻⁷ Torr, ions may not besufficiently scattered, and thus an active material based on a metal, anactive material based on a metal oxide, or a mixture thereof may not besufficiently deposited.

During the sputtering operation, although not limited, a workingtemperature of a non-woven fabric type current collector is within arange from about 0° C. to about 200° C., may be within a range fromabout 10° C. to about 90° C., and may preferably be within a range fromabout 10° C. to about 80° C.

Furthermore, during the sputtering operation according to the presentinvention, although not limited, flux of introduced argon gas is withina range from about 10 cm³/min to about 50 cm³/min, may be within a rangefrom about 2 cm³/min to about 30 cm³/min, and may preferably be fromabout 5 cm³/min to about 20 cm³/min.

Although the above-stated plasma reactor relates to a capacitivelycoupled reactor, it is merely an example, and the present invention isnot limited thereto. For example, the plasma reactor may have anotherplasma source, such as an inductively coupled plasma source, a magnetronplasma source, and an electromagnetic resonance, and, if necessary, mayinclude a remote plasma source.

FIGS. 4A to 4C are diagrams showing stages of growth of a lithiatedlayer 100L that an active material formed on a metal fiber in anon-radial shape experiences as being lithiated during a chargingoperation according to an embodiment of the present invention, and FIG.4D is a graph showing change of stresses in the respective stages ofgrowth. The horizontal axis indicates charging times corresponding tolithium insertion, whereas the vertical axis indicates stresses.

Referring to FIGS. 4A to 4C, considering a stage A in which lithiationbegins on a surface of an active material layer with a circularity from0.2 to 0.8, a representative stress element SE located in an activematerial layer core 100P that is not yet lithiated experiences smalltensile stress due to the expanding lithiated layer 100L. As lithiationproceeds as in a stage B, compression stress is applied to therepresentative stress element SE located in front of lithiation movingtoward the active material layer core 100P. However, even the lithiationproceeds to a stage C, compression stress is still applied to therepresentative stress element SE located in the lithiated layer 100L,where the corresponding region is a region at which the lithiated layer100L still experiences an elastic behavior with respect to thecompression stress. Therefore, no crack or fracture occurs in thelithiated layer.

Referring to FIG. 4D, the dominant stress change in the active materiallayer shown in the graph includes tensile stress SA in the stage A andcompression stress SB in the stage B. In the stage C where the lithiatedlayer 100L is dominant, compression stress SC1 or subtle tensile stressSC2 is applied.

At a circularity from 0.2 to 0.8, compression stress comp is applied toa surface of the lithiated layer 100L on the active material layer core100P. Under the compression stress σcomp, the lithiated layer 100L is aregion experiencing an elastic behavior, and thus no crack or fractureoccurs in the lithiated layer 100L. Even if lithium ions Li+ areomnidirectionally transferred to a surface of an active material layervia an electrolyte absorbed through porosity and the lithiated layer100L grows as a shell, magnitude of tensile hoop stress applied to theshell may be reduced or eliminated throughout the shell due to thecompression stress comp due to the interface between flat surfaceelements based on a controlled circularity. Therefore, formation of acrack on a surface of the lithiated layer 100L may be suppressed.

FIGS. 5A to 5C are diagrams showing stages of growth of a lithiatedlayer 100L that an active material formed on a metal fiber in a radialshape experiences as being lithiated during a charging operationaccording to a comparative embodiment, and FIG. 5D is a graph showingchange of stresses in the respective stages of growth. The horizontalaxis indicates charging times corresponding to lithium insertion,whereas the vertical axis indicates stresses.

Referring to FIGS. 5A to 5C and FIG. 5D, in a stage A where lithiationbegins on a surface of an active material layer according to thecomparative embodiment of which circularity is substantially 1, arepresentative stress element SE located in an active material layercore 100P that is not yet lithiated experiences small tensile stress dueto the expanding lithiated layer 100L as in the above embodiment of thepresent invention. Furthermore, as lithiation proceeds as in a stage B,compression stress is applied to the representative stress element SElocated in front of lithiation moving toward the active material layercore 100P. However, in a stage C, as elastic deformation is graduallyremoved at the representative stress element SE located in the lithiatedlayer 100L, the lithiated layer 100L dominantly grows in a radial shape(or, in radial directions), thereby inducing hoop stress of tensilestress equal to or greater than critical tensile stress σplastic. As aresult, a surface of the lithiated layer 100L weakened due to volumeexpansion is cracked or fractured.

According to an embodiment of the present invention, an active materiallayer is formed to have a reduced circularity from about 0.2 to about0.8, where the formation thereof may be easily controlled by using aplasma-based dry deposition method. By using an electrode including anon-woven fabric type current collector consisting of such a non-radialactive material layer and metal fibers, irreversible reactions based oncracks or fractures of silicon particles that occurs during charging ofa battery may be suppressed or reduced.

FIG. 6 is a sectional view for describing an electrochemical reaction ofa battery cell 500 employing an electrode according to an embodiment ofthe present invention.

Referring to FIG. 6, electrodes with different polarities including ananode electrode 200 and a cathode electrode 300 may be stacked to formthe battery cell 500. Conductive tabs (not shown) may be attached tofirst ends of the electrodes 200 and 300. For insulation between theanode electrode 200 and the cathode electrode 300, a separator 400, suchas a polymer-based micro-porous film, a woven fabric, a non-wovenfabric, a ceramic, an intrinsic solid polymer electrolyte film a gelsolid polymer electrolyte film, or a combination thereof, may bearranged between the electrodes 200 and 300. As an unlimited example, inthe battery cell 500, an electrode containing a salt, such as potassiumhydroxide (KOH), potassium bromide (KBr), potassium chloride (KCL), zincchloride (ZnCl2), and sulfuric acid H2SO4 may be absorbed to theelectrodes 200 and 300 and/or the separator 400, and thus the batterycell 500 may be completed.

An electrode including a non-woven fabric current collector and anactive material stacked on metal fibers thereof in a non-radial shapeaccording to the above-stated embodiment may be applied as any one of orboth the cathode electrode 300 and the anode electrode 200. Preferably,an electrode according to an embodiment of the present invention may beapplied as the anode electrode 200. FIG. 6 shows an embodiment in whichan electrode according to an embodiment of the present invention isapplied as the anode electrode 200.

During a charging operation or a discharging operation, the anodeelectrode 200 may perform ion exchanges as indicated by the arrow byutilizing both the cathode electrode 300 facing the anode electrode 200and the both main surfaces of the anode electrode 200. For example, inthe battery cell 500, the two cathode electrodes 300 may share thesingle-layer anode electrode 200. Therefore, while the battery cell 500is being charged, all of lithium ions of the cathode electrodes 300 movetoward the both surfaces of the anode electrode 200 as indicated by thearrows P1 and P2. Incidentally, while the battery cell 500 is beingdischarged, lithium ions move toward the cathode electrodes 300 indirections respectively opposite to the directions indicated by thearrows P1 and P2.

According to an embodiment of the present invention, compared to anelectrode structure including a metal foil current collector coated withan electrically active material, a smaller number of separators areemployed, and thus energy density may be increased.

Furthermore, since the non-woven fabric type electrode consisting ofmetal fibers and an active material combined with the same may maintainfibrous characteristics, the non-woven fabric current collector may beeasily deformed. Furthermore, since a substantially uniform conductivenetwork is formed throughout the electrode, even if thickness of theelectrode is increased to control capacity of a battery, internalresistance does not increase unlike as in a conventional batterystructure obtained by coating an active material layer onto a metalfoil. Therefore, charging and discharging efficiencies may be maintainedor improved, and thus a high capacity battery may be provided.

Furthermore, due to ease of deformation or elastic characteristics basedon fibrous characteristics of an electrode according to an embodiment ofthe present invention, the electrode may not only be formed as a woundtype, but also be 3-dimensionally deformed by being stacked, bent, andwound, and thus the electrode may be applied to batteries having shapesother than a cylindrical shape, e.g., a hexahedral shape, a pouch-likeshape, etc., or batteries having any of various volumes and shapes to beintegrated with fabric products like clothes and bags. Furthermore, anelectrode according to an embodiment of the present invention may beapplied to a flexible battery with excellent bending characteristics forwearable devices.

Hereinafter, detailed embodiments of the present invention will be givenbelow. However, the embodiments described below are disclosed merely asexamples of the present invention, and the present invention is notlimited thereto.

First Embodiment

A non-woven fabric current collector including metal fibers, which areformed of an alloy of iron, nickel, and chromium and have an averagediameter of 10 was arranged at a location inside a plasma reactor, and asputtering target formed of silicon was arranged at another locationinside the plasma reactor to face the non-woven fabric currentcollector. A plasma source for deposition of an active material layerwas based on RF capacitive coupling, a working temperature was 10⁻⁶Torr, a working temperature was 25° C., and a working time was 2 hours,where a anode electrode including the non-woven fabric current collectorhaving deposited thereon a silicon layer was fabricated by injectingargon gas at the flux of 15 cm³/min. FIG. 7 is a diagram showing asilicon active material layer 100 deposited onto the metal fiber 10W ina non-radial shape. Here, circularity of the silicon active materiallayer 100 was within a range from 0.2 to 0.8, that is, about 0.46.

A battery was fabricated in a glove box with the argon gas atmosphere byusing the electrode fabricated as described above and electrochemicalcharacteristics of the battery were evaluated.

First Comparative Embodiment

An electrode and a battery were fabricated under the same conditionsusing the same method as those of the first embodiment except that acopper thin-film current collector having a same size was employedinstead of the non-woven fabric current collector consisting of metalfibers formed of an alloy of iron, nickel, and chromium, andelectrochemical characteristics of the battery were evaluated.

For evaluation of characteristics of the batteries, dischargingcharacteristics measured by applying constant currents of 300 mA/g tothe battery according to the first embodiment and the battery accordingto the first comparative embodiment are shown in FIGS. 8A and 8B,respectively.

Referring to FIGS. 8A and 8B, battery capacity of the battery accordingto the first embodiment was more than 1.4 times greater than that of thebattery according to the first comparative embodiment. Furthermore, incase of the battery fabricated according to the first embodiment,discharge amount of 3000 mAh/g was maintained well even if the batterywas charged and discharged 5 times. However, in case of the batteryfabricated according to the first comparative embodiment, dischargeamount was reduced from 2,500 mAh/g to 2,300 mAh/g as the battery wascharged and discharged once (1st), three times (3rd), and five times(5th).

Furthermore, for evaluation of life expectancy characteristics of thebatteries, each of the battery according to the first embodiment and thebattery according to the first comparative embodiment was charged anddischarged 200 times by applying currents of 2,000 mA/g thereto, andresults thereof are shown in FIG. 9. Referring to FIG. 9, in case of thebattery according to the first embodiment, capacity of the battery afterbeing charged and discharged was maintained to 90% of the initialcapacity thereof. Incidentally, capacity of the battery according to thefirst comparative embodiment was significantly reduced before beingcharged and discharged 10 times.

FIGS. 10A and 10B are scanning electron microscope images of the batteryaccording to the first embodiment before being charged/discharged andafter being charged/discharged. Referring to FIGS. 10A and 10B, anactive material layer was well maintained without an exfoliation in thebattery according to the first embodiment. FIG. 11 is an optical imageof the battery according to the first comparative embodiment beforebeing charged/discharged and after being charged/discharged. Referringto FIG. 11, unlike the battery according to the first embodiment, anelectrode layer is exfoliated from a current collector as the batterywas charged and discharged.

Second Embodiment

An electrode and a battery were fabricated under the same conditionsusing the same method as those of the first embodiment except that aFe₂O₃ active material was used instead of a silicon active material, andelectrochemical characteristics of the battery were evaluated.

Second Comparative Embodiment

An electrode and a battery were fabricated under the same conditionsusing the same method as those of the first embodiment except that acopper thin-film current collector having a same size was employed, andelectrochemical characteristics of the battery were evaluated.

As a result, in case of the battery according to the second embodiment,capacity of the battery after being charged and discharged 200 times wasmaintained to 90% of the initial capacity thereof. Incidentally,capacity of the battery according to the second comparative embodimentwas significantly reduced before being charged and discharged 10 times.Furthermore, shape of the battery according to the second embodiment waswell maintained compared to the initial shape of the battery. However,in the battery according to the second comparative embodiment, anelectrode layer is exfoliated from a current collector as the batterywas charged and discharged.

Third Embodiment

An electrode and a battery were fabricated under the same conditionsusing the same method as those of the first embodiment and the secondembodiment except that active material layers were deposited for 0.5hours, 2 hours, and 10 hours to evaluate characteristics of the battery,evaluate life expectancy characteristics of the battery, and checkchanges of shape of the electrode according to working times, andelectrochemical characteristics of the battery were evaluated under thesame condition as those in the first embodiment and the secondembodiment.

As a result, in case of the battery according to the third embodiment,capacity maintained after charging and discharging the battery 200 timesgradually decreased as the working time of the sputtering operationincreased. Particularly, when an active material was deposited for 2hours or less, about 90% of the initial capacity was maintained.However, when an active material was deposited for 10 hours, about 10%of the initial capacity was maintained (refer to Table 1). The reasonthereof is that, since working time for deposition of an active materialincreases, an amount of deposited active material increases, and thusvolume expansion at the lower portion of the deposited active materialmay not be effectively buffered. As a result, it is difficult tomaintain capacity.

TABLE 1 Si Active Material Fe₂O₃ Active Material Initial After 200Initial After 200 Capacity Cycles Capacity Cycles (mAhcm⁻²) (mAhcm⁻²)(mAhcm⁻²) (mAhcm⁻²) 0.5 Hours 0.088 0.087 0.059 0.057   2 Hours 0.3540.318 0.209 0.183  10 Hours 1.770 1.181 1.138 0.147

While the present invention has been particularly shown and describedwith reference to embodiments thereof, it will be understood by those ofordinary skill in the art that various changes in form and details maybe made therein without departing from the spirit and scope of thepresent invention as defined by the following claims

INDUSTRIAL APPLICABILITY

According to an embodiment of the present invention, by depositing anactive material layer onto metal fibers of a non-woven fabric currentcollector including the metal fibers, which form porosity, in anon-radial shape by operingusing a plasma-based sputtering, an electrodefor suppressing tensile stress formed during operations for charging anddischarging a high capacity active material with a large volumeexpansion ratio to suppress irreversibility, such as exfoliation of theactive material, and to effectively increase a ratio of specific surfacearea to volume for increased energy density may be provided.

Furthermore, according to another embodiment of the present invention, amethod of fabricating an electrode for a secondary battery, thebinder-free method for dry-fabricating an electrode including anon-woven fabric current collector having formed thereon an activematerial layer in a non-radial shape may be provided.

What is claimed is:
 1. An electrode for a secondary battery comprising:a non-woven fabric current collector including metal fibers that formcontinuous porosity from a surface of the non-woven fabric currentcollector to the interior of the non-woven fabric current collector; andan active material layer deposited onto the metal fibers in a non-radialshape through the porosity in a plasma-based sputtering operation. 2.The electrode for a secondary battery of claim 1, wherein the activematerial layer deposited in the non-radial shape has a circularity,which is defined by Equation 1 below, from 0.2 to 0.8; $\begin{matrix}{{Circularity} = \frac{2\sqrt{\pi \; A}}{P}} & \left\lbrack {{Equation}\mspace{14mu} 1} \right\rbrack\end{matrix}$ (A denotes an entire area of a cross-section of the metalfiber and the active material layer formed on the metal fiber, and Pdenotes circumferential length of the cross-section).
 3. The electrodefor a secondary battery of claim 1, wherein the cross-section has anelliptical shape.
 4. The electrode for a secondary battery of claim 1,wherein the active material layer is deposited from a surface of thenon-woven fabric current collector to the interior of the non-wovenfabric current collector.
 5. The electrode for a secondary battery ofclaim 1, wherein size of the porosity is equal to or larger than that ofthe sheath of the plasma.
 6. The electrode for a secondary battery ofclaim 1, wherein size of the porosity is within a range from about 0.01mm to about 2 mm.
 7. The electrode for a secondary battery of claim 1,wherein diameter of the metal fiber is within a range from about 1 μm toabout 200 μm.
 8. The electrode for a secondary battery of claim 1,wherein the metal or the metalloid is any one selected from a groupconsisting of tin (Sn), silicon (Si), antimony (Sb), zinc (Zn),germanium (Ge), aluminum (Al), copper (Cu), bismuth (Bi), cadmium (Cd),magnesium (Mg), cobalt (Co), arsenic (As), gallium (Ga), lead (Pb), andiron (Fe) or an inter-metallic compound.
 9. The electrode for asecondary battery of claim 1, wherein the metal fiber is formed of astainless steel, iron, aluminum, copper, nickel, chromium, titanium,vanadium, tungsten, manganese, cobalt, zinc, ruthenium, lead, iridium,antimony, platinum, silver, gold, or an alloy thereof.
 10. The electrodefor a secondary battery of claim 1, wherein the metal fibers areacid-cleaned to control surface roughness.